Union of multiple sets idiom broken because member type is not inferred

[cjolowicz]

The idiom set().union(*sets) is used to produce the union of multiple sets:

>>> sets = [{1, 2}, {2, 3}]
>>> set().union(*sets)
{1, 2, 3}

This does not type check with mypy:

from typing import Set


sets = [{1, 2}, {2, 3}]
union: Set[int] = set().union(*sets)


# main.py:5: error: 
#  Incompatible types in assignment (expression has type "Set[<nothing>]", 
#  variable has type "Set[int]")
# main.py:5: error: 
#  Argument 1 to "union" of "set" has incompatible type "*List[Set[int]]"; 
#  expected "Iterable[<nothing>]"

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Apparently, mypy infers the type of set() to be Set[<nothing>]. Ideally, it would work out from the context that set() should have type Set[int]. I would think that an empty set (with no other reference to it) can have any member type, including the one that works in the specific context.

The following code type-checks:

from typing import Set
sets = [{1, 2}, {2, 3}]
zero: Set[int] = set()
union = zero.union(*sets)

[goodmami]

This appears to be a duplicate of #2013. Also I don't think the problem is with the idiom of the union of multiple sets, but just the union on the same line as set instantiation. Here is what I came across:

# error: Incompatible types in assignment (expression has type "Set[str]", variable has type "Set[Optional[str]]")
maybe_strs: Set[Optional[str]] = set('abc').union((None,))
# the following type checks
maybe_strs2: Set[Optional[str]] = set('abc')
maybe_strs2.add(None)

edit
I just noticed that using the | operator instead of union type checks fine:

maybe_strs3: Set[Optional[str]] = set('abc') | {None}

[JukkaL]

Closing as duplicate of #2013, as pointed out by @goodmami.